Imaging systems designed to produce digital images from a capture medium such as a photographic film strip can encounter problems with color reproduction due to a variety of causes. If the spectral sensitivities of the film scanner hardware are not well matched to the spectral transmittances of the dye materials used in common film products, the digital pixel values representing a color neutral object, i.e. a spectrally neutral reflective photographed object, will shift in color in a manner that is linearly related to the scene exposure. Other causes of exposure related color reproduction problems include film material contrast mismatches between different color sensing layers and chemical process sensitivity of the film material.
In U.S. Pat. No. 4,279,502, Thurm et al. discloses a method for optical printing devices that includes determining color balanced copying light amounts from photometric data derived directly from the film without the use of film type specific parameter values. In this method, first and second color density difference functional correlation values are established from density values denoting the results of measurements at a plurality of regions of the photographic film strip which includes the original image being copied. These correlation values are then used for determining the copying light amounts for most of the originals on the photographic film strip. The light amounts for originals containing illuminant error or color dominant subjects are selected differently using empirically determined threshold values. To be effective, this method requires the establishment of two different, independent functional relationships that cannot capture the correct correlation among three primary color densities in the original image.
In U.S. Pat. No. 5,959,720 Kwon et al. describe a similar method for optical printing devices that establishes a linear relationship between film exposure and the gray center color. The method disclosed by Kwon et al. includes the steps of individually photoelectrically measuring the density values of the original film material in at least three basic colors at a plurality of regions of the original film material; and establishing a single, multidimensional functional relationship among the at least three basic colors representing an exposure-level-dependent estimate of gray for use as values specific to said length of the original material for influencing the light amount control in the color copying operation.
Both methods disclosed by Thurm et al. and Kwon et al. include deriving digital images from a film material, analyzing the digital images to establish an exposure dependent color balance relationship, and using the exposure dependent color balance relationship to improve the color appearance of photographic prints made by altering the amount of projected light through the film material onto a photographic paper receiver.
The technology described by Kwon et al. is also used to improve the color appearance of photographic prints made in digital imaging systems. In these applications, the pixel values of the digital images derived by scanning the film material are modified for color balance. That is, a triplet of color pixel values representing the gray center of each digital image is calculated using the established multidimensional functional relationship. The triplet of color pixel values is subtracted from all the pixels of the digital image thus changing the overall color balance of the processed digital image. In addition, the multidimensional functional relationship can be used to modify the color appearance of pixels of the digital images on a pixel-by-pixel basis. However, there are still problems associated with Kwon et al.'s technique that relate to the non-linear photo response of the capture medium, in particular to pixels relating to under-exposed regions of the photographic film strip.
In U.S. Pat. No. 5,134,573, Goodwin discloses a method for adjusting the contrast of digital images derived from digitally scanned photographic film materials. The method improves the overall image contrast through the application of a sensitometric correction function in the form of a look-up-table (LUT) designed to linearize the photographic response of photographic film products. While the application of sensitometric correction function does improve the color contrast of the digital image pixel values corresponding to under-exposed regions of photographic film materials, it does not account for the exposure dependent color problems associated with regions of film materials that were exposed with proper amounts of light. Furthermore, Goodwin's method does not account for changes in the film sensitometry induced by chemical processing errors.